NGC 1866 is a young, rich star cluster in the Large Magellanic Cloud.
Since the cluster is
very well populated both in the main sequence and post main sequence
stages, thus providing us with a statistically complete
sample of objects throughout the various evolutionary phases of
intermediate mass stars,
it represents a good laboratory
for testing stellar evolutionary models.
More precisely, NGC 1866 can be used to discriminate among
classical stellar models, in which the extension of the
convective regions is fixed
by the classical Schwarzschild criterion, from models with
overshooting, in which an “extra-mixing” is considered to take
place beyond the classical limit of the convective zone.
Addressing this subject in a recent work, Testa et al. ([CITE]) reached the
conclusion that the classical scheme for the treatment
of convection represents a good and sufficient approximation for
convective interiors.
Using their own data, we repeat here the analysis. First we revise
the procedure followed by Testa et al. ([CITE]) to correct the data for
completeness,
second we calculate new stellar models with updated physical input
for both evolutionary schemes, finally we present many simulations
of the colour-magnitude diagrams and
luminosity functions of the cluster using the ratio of the
integrated luminosity
function of main sequence stars to the number of
giants as the normalization factor of the simulations. We also take
into account several possible physical agents that could alter the
color-magnitude diagram and the luminosity function: they are unresolved
binary stars, dispersion in the age, stochastic effects in the initial
mass function. Their effect is analyzed separately, with the conclusion
that binary stars have the largest impact.
The main result of this study is that
the convective overshoot hypothesis (together with a suitable
percentage of
unresolved binaries) is really needed to fully match the whole
pattern of data. The main drawback of the
classical models is that they cannot reproduce the correct ratio of
main sequence to post-main sequence stars.

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